Method for reducing the sox emission from a plant for manufacturing cement clinker and such plant

ABSTRACT

A description is given of a method as well as a plant for reducing the SO x  emission from a plant for manufacturing cement clinker by which cement raw meal is preheated and burned in a plant comprising a cyclone preheater ( 1 ) and a kiln ( 7 ). The method is peculiar in that a catalyst in the form of a chloride compound and/or a mixture of several chloride compounds is introduced into the preheater ( 1 ) at its uppermost or next-to-uppermost cyclone stage ( 2, 3 ), that the catalyst is directed down through the preheater ( 1 ) to the kiln ( 7 ), that a partial amount of the kiln exhaust gas stream containing the catalyst in vaporized form is extracted from the kiln ( 7 ), that the extracted exhaust gas stream is cooled off so that the catalyst is present in solid form, that solid matter is separated from the cooled exhaust gas stream and that at least some of the separated solid matter containing the catalyst is recirculated for renewed introduction into the preheater ( 1 ). Hereby is obtained an effective reduction of the SO 2  emission. The reason for this is that chloride compounds and/or a mixture of several chloride compounds will promote the reaction catalytically of SO 2  with CaO for formation of CaSO 3  and it will further promote the reaction of SO 2  with CaCO 3  for formation of CaSO 3  and CO 2 . Since CaCO 3  is present in great abundance, the SO 2  reduction may be effected without any use of extraneous chemicals.

[0001] The present invention relates to a method for reducing the SO_(x)emission from a plant for manufacturing cement clinker by which cementraw meal is preheated and burned in a plant comprising a cyclonepreheater and a kiln. The invention also relates to a plant for carryingout the method.

[0002] Plants of the aforementioned kind for manufacturing cementclinker are generally known from the literature.

[0003] The emission of SO₂ from such modern kiln plants formanufacturing cement clinker is normally relatively low due to the factthat the sulphur contained in the fuel input for the kiln and for anycalciner is bound very effectively in the form of sulphate which isdischarged from the kiln embedded in the clinker. However, a certainemission from the preheater may occur if the utilized raw materialscontain sulphide as is the case in the frequently occuring mineralspyrite and marcasite.

[0004] The reason for this is that pyrite FeS₂ is decomposed in thepreheater at temperatures around 550° C. according to the equation:

FeS₂=FeS+S  (1)

[0005] whereafter the evaporated S is immediately converted by burninginto SO₂. FeS is somewhat more resistant and reaches the calcining zonebefore it is burned, and the SO₂ thus generated is subsequently bound byCaO in similar way as that produced from the fuel. This happens inaccordance with the reaction equation:

SO₂+CaO+½O₂=CaSO₄  (2)

[0006] In case of a pyrite content in the raw materials, there is a riskthat about one half of the sulphur content may escape in the form ofSO₂.

[0007] To reduce the extent of any such SO₂ escape, it is known practiceto introduce an absorbent in the form of CaO, Ca(OH)₂ or other basiccomponents at some location in the preheater so that SO₂ can be bound inthe form of sulphite:

CaO+SO₂=CaSO₃  (3)

[0008] At a subsequent stage of the process, sulphite will be convertedinto sulphate.

[0009] A significant disadvantage of this known method is that itinvolves use of a surplus amount of absorbent, making the methodrelatively costly, particularly if the absorbent which is being used hasto be purchased from an external source.

[0010] From WO 93/10884 a method is known by which exhaust gasescontaining CaO-laden dust are extracted from a location near thecalciner and directed to the location in the preheater where theabsorption of SO₂ is to take place. The method works, but it requires aquite substantial surplus amount of CaO, thus somewhat reducing theefficiency of the preheater as a heat exchange unit.

[0011] The Danish patent application No. PA 1999 00867 provides animproved method by which calcined cement raw meal is extracted, slakedand ground before it is introduced into the preheater. However, thismethod also has the disadvantage of reduced efficiency of the preheateras a heat exchange unit.

[0012] The purpose of the present invention is to provide a method aswell as a plant for manufacturing cement clinker by means of which acheap and effective reduction of the SO_(x) emission is achieved withoutentailing a noteworthy reduction in the efficiency of the preheater.

[0013] This is achieved by a method of the kind mentioned in theintroduction, and being characterized in

[0014] that a catalyst in the form of a chloride compound and/or amixture of several chloride compounds, having the property that it willbe present in solid or melted form in the zone of the preheater whereSO₂ is formed, and it thus must be effective, and in vaporized form inthe kiln, is introduced into the preheater at its uppermost ornext-to-uppermost cyclone stage,

[0015] that the catalyst is directed down through the preheater to thekiln,

[0016] that a partial amount of the kiln exhaust gas stream containingthe catalyst in vaporized form is extracted from the kiln,

[0017] that the extracted exhaust gas stream is cooled off so that thecatalyst is present in solid form,

[0018] that solid matter is separated from the cooled exhaust gas streamand

[0019] that at least some of the separated solid matter containing thecatalyst is recirculated for renewed introduction into the preheater.

[0020] Hereby is obtained an effective reduction of the SO₂ emission.The reason for this is the surprising observation that chloridecompounds and/or a mixture of several chloride compounds having theaforementioned properties in respect to melting point and boiling pointwill promote the reaction catalytically according to the equation

CaO+SO₂=CaSO₃  (3)

[0021] and it will further promote the reaction:

CaCO₃+SO₂=CaSO₃+CO₂  (4)

[0022] So, surprisingly, it has proved possible to catalyze that SO₂reacts with calcium carbonate CaCO₃, which, as it is, constitutes about80% of the raw materials. Since one reactant, namely CaCO₃ is present ingreat abundance, the SO₂ reduction may be effected without any use ofextraneous chemicals, and, furthermore, the reaction, and hence the SO₂reduction must be assumed to be approximately complete.

[0023] The plant for carrying out the method according to the inventionis characterized in that it comprises means for introducing a catalystinto the preheater at its uppermost or next-to-uppermost cyclone stageas well as a bypass system comprising means for extracting a partialexhaust gas stream from the kiln, means for cooling the extractedexhaust gas stream, means for separating solid matter from the cooledexhaust gas stream, and means for recirculating at least a part of theseparated solid matter containing the catalyst for renewed introductioninto the preheater.

[0024] Additional characteristics of the plant will be apparent from thedetailed description provided in the following.

[0025] The catalyst may be separately introduced into the preheater.However, it is preferred that the catalyst is mixed with the rawmaterials, preferentially in the raw mill plant, thereby introducing itinto the preheater together with the raw materials. It is also preferredthat the separated solid matter containing the catalyst which isrecirculated for renewed introduction, is mixed with the raw materialsin the raw mill plant. In cases where the amount of recirculatedcatalyst is insufficient, it will be possible to supplement with freshcatalyst.

[0026] Various chloride compounds such as CaCl₂, KCl, NaCl, MnCl₂ andFeCl₃ may be used as the catalyst. The chloride compounds may beseparately used, but in order to attain a catalyst having theappropriate properties, particularly in respect to the melting point, itis preferred that a mixture of different chloride compounds is used.Since SO₂ is essentially formed in the preheater at a temperature above550° C., the melting point of the catalyst should advantageously be lessthan 550° C. at a pressure of 1 atmosphere.

[0027] A number of existing kiln plants incorporate a bypass systemdesigned for venting chloride and alkali metals from the kiln system. Insuch cases, the bypass system will also be used for extracting, coolingand separating kiln exhaust gases which contain the catalyst. However,in this case the separated solid matter will contain constituents suchas Cl, Na and K which are undesirable elements in the cement, and,therefore, only a portion of this material will be recirculated forrenewed introduction into the preheater, and, furthermore, this willmake it necessary to supplement with fresh catalyst.

[0028] The invention will be explained in further details in thefollowing with reference being made to the drawing which isdiagrammatic, with its only FIGURE showing a plant for carrying out themethod according to the invention.

[0029] In the FIGURE is seen a cement manufacturing plant comprising acyclone preheater 1 consisting of the cyclones 2, 3 and 4, a calciner 5with a subsequent separating cyclone 6 and a rotary kiln 7. The plantfurther comprises a clinker cooler 9 for cooling of burned cementclinker, and a duct 11 for conducting preheated cooling air to thecalciner 5. Raw material from a raw mill plant 21 is introduced into anexhaust gas duct 8, which connects the two uppermost cyclones 2, 3 ofthe preheater and it is preheated in counterflow to the exhaust gas onits passage through the three cyclones, whereafter it is calcined in thecalciner 5. From the bottom outlet of the separating cyclone 6 thecalcined raw material is routed to the rotary kiln 7. The exhaust gasfrom the rotary kiln 7 and the calciner 5 is drawn from the calciner 5through the cyclone 6 and up through the preheater 1 by means of a fan10.

[0030] According to the invention a catalyst is supplied in the form ofa chloride compound and/or a mixture of several chloride compounds intothe preheater at its uppermost or next-to-uppermost cyclone stage whichconstitute the zone of the preheater in which SO₂ is formed. Thecatalyst can be separately supplied into the exhaust gas duct 8 via anopening 8 a or in the corresponding exhaust gas duct 12 which connectsthe cyclone 4 with cyclone 3. However, it is preferred that the catalystis supplied to the raw mill plant 21 in order to attain an effectivemixture with the raw materials and that the catalyst is thereby suppliedto the preheater via the opening 8 a intermingled with the rawmaterials.

[0031] Subsequently, the catalyst will be carried, together with the rawmaterials, down through the preheater 1 to the kiln 7. In the preheaterthe catalyst will catalyze the absorption of SO₂ by promoting itsreaction with CaO for formation of CaSO₃ and also its reaction withCaCO₃ for formation of CaSO₃ and CO₂. The formed CaSO₃ will reactfurther to form CaSO₄ which will be discharged from the kiln embedded inthe clinker.

[0032] Having completed its task in the preheater, the catalyst will endup in the rotary kiln where, due to the prevailing kiln temperature ofmore than 1100° C., the catalyst will be present in vaporized form whichis an essential condition for extracting the catalyst from the kilnsystem. This is done by means of a duct 14 by extracting a partialexhaust gas stream which contains the catalyst in vaporized form anddust, and flowing out of the material inlet end of the rotary kiln.

[0033] The extracted exhaust gas stream is then cooled partly byinjecting air into the duct 14 via the air injection means 16 and partlyin a conditioning tower 15 subject to the injection of water, so thatthe catalyst is present in solid form. The catalyst will essentially becondensed to the dust in the form of alkali chloride during the coolingprocess.

[0034] After cooling, solid matter is separated from the cooled exhaustgas stream in a filter arrangement 17 wherefrom the filtered exhaust gasstream, via a fan 18 and a stack 19, is released into the atmosphere,whereas at least a portion of the filtered-off material containing thecatalyst is recirculated to the raw mill plant 21 or alternativelydirectly to the preheater by means of unspecified conveying means 20 forrenewed introduction into the preheater. Here the raw mill plant isshown merely as a box to which is added a number of raw materialcomponents A, B and C and possibly a catalyst.

1. A method for reducing the SO_(x) emission from a plant formanufacturing cement clinker by which cement raw meal is preheated andburned in a plant comprising a cyclone preheater (1) and a kiln (7),characterized in that a catalyst in the form of a chloride compoundand/or a mixture of several chloride compounds, having the property thatit will be present in solid or melted form in the zone of the preheaterwhere SO₂ is formed and it thus must be effective and in vaporized formin the kiln, is introduced into the preheater (1) at its uppermost ornext-to-uppermost cyclone stage (2, 3), that the catalyst is directeddown through the preheater (1) to the kiln (7), that a partial amount ofthe kiln exhaust gas stream containing the catalyst in vaporized form isextracted from the kiln (7), that the extracted exhaust gas stream iscooled off so that the catalyst is present in solid form, that solidmatter is separated from the cooled exhaust gas stream and that at leastsome of the separated solid matter containing the catalyst isrecirculated for renewed introduction into the preheater (1).
 2. A metodaccording to claim 1, characterized in that the catalyst is separatelyintroduced into the preheater (1).
 3. A method according to claim 1,characterized in that the catalyst is mixed with the raw materials,preferentially in the raw mill plant (21), and is introduced into thepreheater (1) mixed with the raw materials.
 4. A method according toclaim 1, characterized in that the separated solid matter containing thecatalyst which is recirculated for renewed introduction, is mixed withthe raw materials in the raw mill plant (21).
 5. A method according toclaim 4, characterized in that fresh catalyst is supplemented.
 6. Amethod according to claim 1, characterized in that the catalystcomprises one or several chloride compounds such as CaCl₂, KCl, NaCl,MnCl₂ and FeCl₃.
 7. A method according to claim 6, characterized in thatthe catalyst has its melting point at less that 550° C. at a pressure of1 atmosphere.
 8. A plant for carrying out the method according to claim1 comprising a cyclone preheater (1) and a kiln (7), characterized inthat it comprises means (8 a) for introducing a catalyst into thepreheater at its uppermost or next-to-uppermost cyclone stage as well asa bypass system comprising means (14) for extracting a partial exhaustgas stream from the kiln, means (15, 16) for cooling the extractedexhaust gas stream, means (17) for separating solid matter from thecooled exhaust gas stream, and means (20) for recirculating at least apart of the separated solid matter containing the catalyst for renewedintroduction into the preheater.